Prevention of cavitation corrosion

ABSTRACT

The present invention directly deals with the problem of cavitation corrosion by modifying the hub of a propeller to eliminate low pressure areas. The present invention is the placement, formation, and/or insertion, on a propeller, of a non-smooth area in each facet of the hub between each of the blades. This area is also sometimes referred to as the “blade root”. The exact configuration of the raised area can vary from a longitudinal raised area to single or multiple bumps or buttons to single or multiple pointed projections. By modifying the hub, whether by casting a new propeller with a redesigned hub, or by retrofitting an existing hub, the elimination of corrosion occurs when a high Developed Area Ratio (DAR) propeller hub has one or more raised protrusions on the surfaces of the propeller hub between each blade seat. The addition of the raised protrusions discourages the formation of low pressure areas around the hub. By removing the low pressure areas from the immediate area around the hub, the shock waves caused by the collapse of the low pressure areas is moved away from the propeller, thus eliminating corrosion impact damage.

CROSS-REFERENCE TO RELATED PATENTS

This application claims the benefit, under Title 35, United States Code, Sec. 119(e), of U.S. provisional Application No. 60/636,176 filed Dec. 14, 2004, titled PREVENTION OF CAVITATION CORROSION, the disclosure of which, including all appendices and all attached documents, is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to design of propulsion devices, more specifically to the design of high efficiency propellers.

BACKGROUND OF THE INVENTION

Cavitation is the formation of partial vacuums in a liquid by a swiftly moving solid body or by high intensity sound waves. These partial vacuums are caused by a low pressure in the liquid. When the local ambient pressure at a point in the liquid falls below the liquid's vapor pressure, the liquid undergoes a phase change to a gas, creating “bubbles” or, more accurately, partial vacuums in the liquid.

When the vacuums of cavitation collapse, they create large amounts of noise. The collapse also produces very high energy and can cause major damage. A solid body can become pitted and worn away due to the repeated collapse of these vacuums. This is known as “cavitation corrosion” and is sometimes simply referred to as “cavitation”.

Cavitation corrosion occurs frequently on propellers, such as those used by aircraft and marine vessels. As a propeller's blades move through a fluid, low pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around the propeller becomes. As the pressure reaches vapor pressure, the fluid vaporizes and forms small vacuums. When the vacuums collapse, they typically cause strong shockwaves in the fluid, which can cause damage to the blades or the propeller's hub. This is particularly troublesome in high speed propellers, as the damage to the propeller reduces overall efficiency.

Prior techniques to reduce cavitation corrosion mainly involve keeping the hub of a propeller as smooth as possible between the blades on the blade roots. However, this approach does not completely solve the problem as the partial vacuums still can form around the hub and cause pitting and damage to the hub. What is needed is a way to remove the low pressure areas away from the propeller hub, eliminating the partial vacuums at the hub surface, and thereby eliminating the cavitation of the propeller hub.

SUMMARY OF THE INVENTION

The present invention directly deals with the problem of cavitation corrosion by modifying the hub of a propeller to eliminate low pressure areas. The present invention is the placement, formation, and/or insertion, on a propeller, of a non-smooth area in each facet of the hub between each of the blades. This area is also sometimes referred to as the “blade root”. The exact configuration of the raised area can vary from a longitudinal raised area to single or multiple bumps or buttons to single or multiple pointed projections.

By modifying the hub, whether by casting a new propeller with a redesigned hub, or by retrofitting an existing hub, applicant has discovered that the elimination of corrosion occurs when a high Developed Area Ratio (DAR) propeller hub has one or more raised protrusions on the surfaces of the propeller hub between each blade seat. The addition of the raised protrusions discourages the formation of low pressure areas around the hub. By removing the low pressure areas from the immediate area around the hub, the shock waves caused by the collapse of the low pressure areas is moved away from the propeller, thus eliminating corrosion impact damage.

The present invention includes multiple approaches that teach away from the previously held view that the hub and/or blade roots should be as smooth as possible by teaching multiple solutions for accomplishing the inclusion of raised protrusions on the hub and/or blade roots to disrupt the flow of fluid over the hub. In a preferred embodiment, raised bumps are inserted in each blade root of a high speed propeller. The bumps could be round head screws directly inserted into the hub. The bumps can also be a raised brass nub, or bullet, inserted into a machined hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a three blade right hand rotation propeller.

FIG. 2 illustrates a three blade right hand rotation propeller fitted with an embodiment of the present invention.

FIG. 3 illustrates additional views of the propeller illustrated in FIG. 2.

FIG. 4 illustrates a three blade right hand rotation propeller fitted with an additional embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a three blade right hand rotation propeller 100. Blade 105 is in the area of blade root 115. In the normal operation of a high speed propeller, low pressure areas, or partial vacuums, develop around hub 110. As the partial vacuums collapse, shock waves are sent out from the center of the collapsed vacuum. These shock waves reflect off of hub 110 and can cause cavitation corrosion, particularly in the area of blade root 115.

FIG. 2 illustrates a similar propeller to that of FIG. 1, a three blade right hand rotation propeller 200. However, in FIG. 2, the propeller is fitted with an embodiment of the present invention including a disruption means. Raised nubs 220 are integrated into blade root 215. The term integrated is hereby defined as to form, coordinate, or blend into a functioning or unified whole, meaning the disruption means may be cast along with the propeller to form a unified body, or the disruption means may be individual pieces brought together with a previously cast propeller to form a unified body. When blade 205 is turning, nubs 220 acts as the disruption means to alter the path of the fluid around hub 210. This disruption eliminates the partial vacuums from forming around hub 210. Since it is the collapse of these partial vacuums that causes the cavitation corrosion, damage to the hub 210 is eliminated. In a preferred embodiment, a ⅝ inch diameter nub is used, which protrudes ⅜ inch above the blade root. These dimensions, however, are used only as examples and the nubs used in alternate embodiments of the present invention are not limited to the above mentioned dimensions.

FIG. 3 illustrates a front and side view of the propeller from FIG. 2. In these views, the placement of nubs 220 can be seen from different angles. The exact placement is not critical when fitting the nubs onto the propeller, nor is the height of the nubs. The critical factor is that the nubs be positioned such that the fluid flow over the blade roots 215 at the propeller hub 210 is disrupted. And, as discussed before, this disruption of flow eliminates the low pressure areas which cause cavitation.

In the preferred embodiment of the present invention, raised nubs are used as the disruption means on the blade roots of the propeller. These nubs can vary in material and installation technique. One or more screws can be inserted into the propeller leaving the screw heads above the surface of the propeller to form a raised nub. A threaded hole may be machined into the hub, and a threaded nub, or bullet, can be turned into the treaded hole. This bullet can be made of many materials, including but not limited to anodized aluminum and brass stock. With both of the above examples, the height of the nubs can be varied as well.

FIG. 4 illustrates three blade right hand rotation propeller 400 fitted with an additional embodiment of the present invention. In this embodiment, raised ridge 420 is machined into the hub 410 near blade root 415. When blades 405 are turning, this ridge acts similarly to the nubs of the above example by disrupting the water flow of the hub thus reducing the areas of low pressure.

Other configurations of the disruption means will be readily apparent to those of ordinary skill in the art. These can include, but are not limited to, having smaller linear stretches of raised areas, having angled protrusions for disrupting the fluid flow, and even including small, crescent shaped depressions in the hub to cause a disruption to a fluid flow. The methods of equipping and/or manufacturing the propellers to incorporate the disruption means are numerous and include, but are not limited to, machining, retro fitting, casting, and the like.

While several embodiments of the present invention have been disclosed, these embodiments have been presented by way of example only, and are not intended to limit the scope of the present invention. Accordingly, the breadth and scope of the present invention should be defined only in accordance with the following claims and their equivalents. 

1. A propulsion apparatus for propelling an object through a fluid, said apparatus comprising: a hub; a plurality of blades connected to said hub, each of said plurality of blades connected to said hub at a blade root; and disruption means integrated into each blade root, said disruption means altering the flow of said fluid over said hub thereby reducing areas of low pressure around said hub.
 2. The apparatus of claim 1, wherein said disruption means are integrated into each blade root by one of the following techniques: casting, machining, and retro-fitting.
 3. The apparatus of claim 2, wherein said disruption means comprise one or more raised nubs.
 4. The apparatus of claim 3, wherein said one or more raised nubs are of variable height.
 5. The apparatus of claim 2, wherein said disruption means comprise at least one of a raised linear ridge, a series of raised linear ridges, screws, threaded bullets, angled protrusions and crescent shaped depressions.
 6. A method of disrupting fluid flow around a hub of a propeller so as to minimize cavitation damage to said propeller, the method including the step of integrating disruption means into said propeller so as to alter the flow of said fluid around said hub.
 7. The method of claim 6, wherein said propeller comprises: a hub; and a plurality of blades connected to said hub, each of said plurality of blades connected to said hub at a blade root.
 8. The method of claim 7, wherein said step of integrating disruption means into said propeller comprises integrating said disruption means into each blade root.
 9. The method of claim 8, wherein said disruption means are integrated into each blade root by one of the following techniques: casting, machining, and retrofitting.
 10. The method of claim 9, wherein said disruption means comprise one or more raised nubs.
 11. The method of claim 10, wherein said one or more raised nubs are of variable height.
 12. The method of claim 9, wherein said disruption means comprise at least one of a raised linear ridge, a series of raised linear ridges, screws, threaded bullets, angled protrusions and crescent shaped depressions.
 13. A method of modifying a hub of a propeller to minimize cavitation damage to said propeller, the method including the step of adding disruption means to said propeller, said disruption means integrated into said propeller so as to alter the flow of said fluid around said hub.
 14. The method of claim 13, wherein said propeller comprises: a hub; and a plurality of blades connected to said hub, each of said plurality of blades connected to said hub at a blade root.
 15. The method of claim 14, wherein said step of adding disruption means into said propeller comprises integrating said disruption means into each blade root.
 16. The method of claim 15, wherein said disruption means are integrated into each blade root by one of the following techniques: casting, machining, and retro-fitting.
 17. The method of claim 16, wherein said disruption means comprise one or more raised nubs.
 18. The method of claim 17, wherein said one or more raised nubs are of variable height.
 19. The method of claim 16, wherein said disruption means comprise at least one of a raised linear ridge, a series of raised linear ridges, screws, threaded bullets, angled protrusions and crescent shaped depressions. 